11.5: Coral Reefs and Atolls

Reef-building corals are communities of microscopic animals that have photosynthetic dinoflagellates, called zooxanthellae, living symbiotically within their tissues. These reef-building corals, unlike many other corals, cannot grow successfully without the zooxanthellae, or where there is not enough light for the zooxanthellae to photosynthesize. Reef-building corals also require warm water (≥18°C) and are intolerant of low salinity and high concentrations of suspended sediments that block sunlight. Hence, coral reefs grow only in shallow tropical and subtropical waters that are not subject to low salinity and high turbidity caused by stream runoff or abundant plankton.

The three basic types of coral reefs are fringing reefs, barrier reefs, and atolls (Fig. 4-28). Fringing reefs grow in shallow waters along the shore and are best developed off coasts with arid climates and where river runoff is limited. Fringing reefs are distributed in a patchy fashion along most coasts where they occur because they are easily damaged by the low salinity of excess freshwater runoff and by high concentrations of suspended sediment in the runoff or resuspended by storm waves. Human activities such as flood-control projects, dredging, sewage discharge, and coastal modification often result in elevated concentrations of suspended sediments and enhanced runoff of low-salinity water during storms. Consequently, fringing coral reefs are damaged by these activities in many locations, but it is often difficult to distinguish damage due to human influence from natural variations.

Barrier reefs and atolls are formed only where there has been a change of sea level on the adjacent coast. If sea level rises or the coast subsides in an area that has a fringing reef, the reef grows upward. The upward growth is faster at some distance offshore where conditions are optimal. In the shallowest water close to the shoreline, wave resuspension of sediments and the effects of runoff inhibit coral growth. In deeper offshore water, reduced light levels inhibit photosynthesis by zooxanthellae and thus coral growth. Therefore, the optimal zone for coral growth is a strip of ocean some distance offshore, but not extending into deep water. The width of this zone varies with factors such as the seafloor slope and the amount of runoff. As the land subsides, the reef grows upward in this zone faster than coral can grow in either shallower or deeper water, and a barrier reef is formed (Fig. 4-29b) Once formed, the barrier reef continues to grow upward. The rate of upward growth is usually just sufficient to match the changing sea surface level. The top of the reef remains just below the water surface, because corals are damaged if exposed to air for prolonged periods. If the maximum possible rate of upward growth is less than the rate at which the land subsides, the reef is submerged and eventually dies.

Once a barrier reef is formed, it closes or partially closes a lagoon between the reef and the coastline. Corals grow in this lagoon, but they are limited by higher turbidity due to land runoff, by salinity variations, or by lack of nutrients as a result of the long residence time of the lagoon water (Chap. 12, CC8).

Barrier reefs are especially abundant on the Earth at present because sea level has been rising eustatically for approximately the past 19,000 years. The Great Barrier Reef of Australia, which is about 150 km wide in places and more than 2000 km long, is the best-known barrier reef.

Islands formed by volcanoes on oceanic crust are particularly good locations for the development of barrier reefs. Once a volcanic island has moved away from the oceanic ridge or hot spot on which it was formed, it cools and sinks isostatically (Chap. 4, CC2). Hence, even in times of falling sea level, barrier reefs can form around such islands, as long as the island sinks faster than the sea level falls. Sinking of volcanic islands leads to the formation of the third type of coral reef, the atoll (Figs. 4-29c, 4-30). As the island sinks, a barrier reef develops from the fringing reef and continues to grow upward, remaining at or close to the sea surface as the island continues to sink. Eventually the island sinks entirely below the ocean surface, and its reef is left in the form of an atoll. The atoll continues to grow and maintain itself as the island sinks if the coral growth rate exceeds the combined sinking rate of the island plus the rate of any sea level rise.

Low islands may form on an atoll, particularly on the side of the atoll from which wind and waves most often approach. Such islands are composed of debris from the reef and of calcareous algae that grow in very shallow water and can survive exposure at low tide. Islands can also be formed on an atoll by earthquakes that uplift or tilt the sinking volcano.